1. Field of the Invention
The present invention relates generally to fiber optic couplers. More particularly, the present invention relates to polarization insensitive fused fiber optic couplers.
2. The Prior Art
Fiber optic couplers including a biconical tapered coupling region, three or four terminal ports, and a housing to maintain the coupling region in a stable environment are known and have been promoted in various forms. For instance, fiber optic couplers formed by fusing a pair of side-by-side optical fibers by heating and stretching the optical fibers and forming biconical tapered regions have been explored to certain limits. Such experimentation has resulted in light couplings, the utility of which is a function of the mode of the fiber and the length and shape of the coupling region. These prior art couplings suffer from polarization accumulation due to the consistently oriented elliptical shape of the cross-section of the fused fibers in the coupling region which may not only cause polarization dependent loss (PDL), but may also cause an envelope modulation of the light transmission spectrum in Wavelength Division Multiplexers (WDWs). Although it has been discovered that smaller channel spacing of WDMs may be achieved by increasing the length of the tapered section of the fused region, these devices have a tendency to also have a greater sensitivity to the input state of polarization.
Attempts to overcome such problems have resulted in subsequent twisting of the fused fibers after heating and stretching of the fibers such as reported by J. Wilkinson and C. J. Rowe in xe2x80x9cClose-Spaced Fused Fibre Wavelength Division Multiplexers with Very Low Polarisation Sensitivity,xe2x80x9d Electronic Letters, Vol. 26, pp. 382-384 (1990). However, U.S. Pat. No. 5,408,555 to Fielding et. al. discussing a related, if not substantially similar, subject, seems to somewhat disparage the Wilkinson/Rowe findings. In particular, the Fielding patent takes issue with the Wilkinson/Rowe findings regarding the relationship of twisting on polarization and wavelength shifts.
Both sources seem to agree that the twisting step, properly applied may decrease the polarization sensitivity experienced previously. However, twisting produces undesirable bending stress and increases the potential for mechanical unreliability. Additionally, it is very difficult to adjust the output signals of the two fibers to a maximum and minimum respectively at a single wavelength when fabricating WDMs. Furthermore, the window spacing cannot be monitored when fabricating interleaving WDMs, or for example, DWDMs.
To overcome these and other shortcomings of the prior art, disclosed herein is a technique for forming a polarization insensitive fused fiber optic coupler and a coupler resulting therefrom. The present invention provides a fused coupler in which two optical fibers are fused together and elongated to form a biconical taper for optical power coupling. The cross-sectional shape of the tapered region is elliptical thus producing birefringence. Due to the difference in coupling coefficients between two orthogonal polariztions, fused couplers generally exhibit polarization dependent loss (PDL). In order to form a polarization insensitive coupler, instead of elastically twisting the coupler after the fusing and elongating process, it has been found that non-elastic twisting can be accomplished by twisting the fibers in the coupling region during the fusing and elongating process or alternately twisting and heating the coupling region after the fusing and elongating process resulting in elimination of Polarization Dependent Loss (PDL). Furthermore, by twisting during fusing, complications associated with bending stresses previously known for twisting after fusing, is also substantially, if not completely, eliminated.
It is therefore an object of the present invention to produce a polarization insensitive fiber optic coupler that is also more resistant to bending stresses than previously known heretofore.
It is also an object of the present invention to produce a fiber optic coupler capable of withstanding greater fiber twisting as compared to the prior art coupling devices.
It is another object of the present invention to provide a method for forming the novel polarization insensitive fiber optic coupler.
It is yet another object of the present invention to provide a WDM and interleaving WDM that is polarization insensitive and envelope modulation free in the optical spectrum.
Viewed from a first vantage point a fiber optic coupler is disclosed, comprising in combination, a fused, stretched, and twisted biconical tapered region; a first pair of fiber pigtail ends extending from a first end of the biconical tapered region; a second pair of fiber pigtail ends extending from a second end of the biconical tapered region; and polarization dependent loss preclusion means disposed within the biconical tapered region.
Viewed from a second vantage point a method for forming a polarization insensitive fiber optic coupling is disclosed, comprising in combination, stripping cladding from a central region of each of two fibers; attaching the fibers to fiber holders and placing the fibers side by side; connecting a first end of one of the fibers to a light source and connecting a second end of that same fiber to a monitoring means; twisting the fibers about one another in one turn; substantially simultaneously heating and stretching the central stripped regions of the fibers; twisting the fibers about each other upon detection of light coupling via said monitoring means; discontinuing heating, stretching, and twisting upon detection of a predetermined coupling performance via the monitoring means; and trimming the coupling region.
Viewed from a third vantage point a fiber optic coupler, comprising in combination, a first pair of fiber optic pigtail ends; a second pair of fiber optic pigtail ends; a fused biconical tapered and twisted region extending between the first and second pair of pigtail ends; and a polarization insensitive coupling region within the tapered region.